The inhibition of Phytophthora root rot of
avocado with potassium silicate application
under greenhouse conditions
T F Bekker1, N Labuschagne2, T Aveling2 and C Kaiser1
Department of Plant Production and Soil Science
Department of Microbiology and Plant Pathology
University of Pretoria, Pretoria, South Africa
1
2
ABSTRACT
Phytophthora cinnamomi root rot causes extensive avocado tree root death. The use of phosphonate fungicides is currently the only
effective post-infectional control method, and to this end, an alternative was sought to inhibit Phytophthora root rot. Four replications
were conducted over a period of two years to determine the efficacy of potassium silicate in inhibiting Phytophthora root rot in avocado nursery trees. Treatments consisted of uninoculated and inoculated untreated trees; uninoculated, silicon treated trees; trees
inoculated with Phytophthora cinnamomi inoculum treated once before or multiple times after inoculation with silicon; and inoculated,
potassium phosphonate (Avoguard®) treated trees. Silicon treated, inoculated trees resulted in the highest fresh and dry root mass
compared to all other treatments. This implies that silicon stimulates growth under infectious stress conditions if applied prior to P.
cinnamomi inoculation. Silicon application did not have a significant effect on canopy condition under conditions of root infection. Root
rot in trees treated with silicon was statistically comparable to root rot in uninoculated, untreated control trees, with higher ratings of
root regeneration / new root formation. Trees receiving one silicon application one day before inoculation, harvested 23 weeks after
inoculation, did not prove to inhibit Phytophthora root rot effectively, as no significant differences were obtained when compared to
the uninoculated, untreated control. Trees receiving one application of silicon but harvested 40 days later had less severe root rot
compared to the uninoculated, untreated trees. This indicates the necessity of reapplication of silicon. Timing of reapplication will be
determined by soil structure, as silicon leaches easily, deeming the applied silicon as unreachable for plant uptake. Sandy soil will
therefore require more regular applications of silicon to maintain the level of resistance required in the host plant. Root rot rating of inoculated trees treated with silicon were in all experiments either statistically comparable to, or better than root rot rating in inoculated
trees, treated with potassium phosphonate. These findings are of paramount importance as this implies that potassium silicate may
be proposed as a possible alternative control to inhibit the effects of P. cinnamomi on avocado trees.
INTRODUCTION
Phytophthora cinnamomi Rands. is a plant pathogen of global
significance as it affects wild and cultivated plants, and is a serious threat to the diversity and structure of natural ecosystems
(Wills and Keighery, 1994). This aggressive fungus causes extensive root rot in avocados (Persea Americana Mill.), and on
average leads to an annual loss of 10% of the world avocado
crop, which amounts to several million US$ worldwide (Zentmyer
& Schieber, 1991).
Although numerous strategies have been implemented to
inhibit Phytophthora root rot, including planting resistant rootstocks (Coffey, 1987; Cahill et al., 1993; Pegg et al., 2002) and
biological control (Pegg, 1977; Casale, 1990; Duvenhage and
Kotze, 1993), chemical control is still the determining factor to
ensure effective inhibition of Phytophthora root rot. Phosphonate
fungicides, including fosetyl-Al and its breakdown product, phosphorous acid, are highly mobile in plants (Guest et al., 1995),
and are believed to control Phytophthora spp. by a combination of direct fungitoxic activity and stimulation of host defense
mechanisms (Guest et al., 1995; Hardy et al., 2001). Duvenhage
(1994) first reported on the possibility of resistance and found
that isolates of P. cinnamomi obtained from trees treated with
fosetyl-Al or H3PO3 were less affected by fosetyl-Al and H3PO3 in
vitro compared to isolates obtained from untreated trees. They
concluded that the possibility of resistance exists, and that the
mode of action is to be determined to effectively prevent this
tendency. It is therefore imperative to obtain new control methods to ensure alternative treatments to be implemented in an
alternative control strategy to limit the possibility of resistance to
develop.
It is commonly accepted that plants need 16 essential nutrient
elements to complete their life cycle (Arnon and Stout, 1939).
Epstein (1999), however, termed silicon to be quasi-essential,
as although plants can complete their life cycle without silicon,
soluble siliceous materials impart numerous beneficial effects to
plants. Soluble silicon in the soil solution is commonly found as
monosilicic acid Si(OH)4, which is easily taken up by plant roots
(Epstein, 1994, 1999, 2001). Silicon occurs in living organisms
as amorphous silica (SiO2 nH2O) and to a lesser extent, soluble
silicic acid, the soluble form taken up by plant roots (Fawe et al.,
1998; Chen et al., 2000). Although the physiological and nutritional role of silicon appears to be limited, evidence is accumulating that silicon absorption has numerous benefits for the plant,
and in particular, plant protection. Inconsistent results have been
found between different studies on different species where prophylactic properties are concerned. Cucumber (Cucumic sativus
L.), rose (Rosa spp.), sugarcane and rice (Oryza spp.) have,
however, received much attention and have been shown to benefit from the application of soluble Si, which leads to disease
protection and consequent higher yields (Bowen et al., 1995).
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
49
The aim of this study was to evaluate whether the addition
of soluble silicon as potassium silicate to P. cinnamomi inoculated avocado nursery trees would inhibit fungal infection, and
possibly increase plant resistance by activating plant defence
mechanisms.
MATERIALS AND METHODS
Chemicals
Silicon was obtained from Ineos Silicas (Pty) Ltd. and potassium
phosphonate (Avoguard®) from Ocean Agriculture, Johannesburg, South Africa.
Experimental detail
Four replicate greenhouse experiments were conducted over a
period of two years to determine the efficacy of potassium silicate in inhibiting Phytophthora root rot in avocado nursery trees.
Avocado nursery trees used in the study were screened for the
absence of Phytophthora cinnamomi by plating out randomly
selected root tips on PARPH (Pimaracin-Ampicillin-RifampicinPentachloronitrobenzene-Hymexazol) medium selective for
Phytophthora (Jeffers & Martin, 1986) and identifying any fungal
growth microscopically. Trees were thereafter sorted on greenhouse benches and treatments assigned according to a randomized block design.
Experiment 1
Twelve-month-old clonal ‘Hass’ on ‘Edranol’ seedling avocado
rootstocks from Allesbeste Nursery (Magoebaskloof, South Africa) grown in composted pine-bark medium were replanted in 5 L
plastic pots in steam-sterilized soil acquired from the University
of Pretoria experimental farm (Pretoria, South Africa) and allowed to re-establish for two months before the experiment was
initiated. Soil texture was 64.9% coarse sand, 13.8% silt and
21.3% clay. The soil pH was 6.3 with 1500 ohm resistance and
the chemical composition was 4 mg.kg-1 P, Bray I; 9703 mg.kg-1
Ca; 533 mg.kg-1 K; 2783 mg.kg-1 Mg; 393 mg.kg-1 Na; 9 mg.kg-1
Cu; 83 mg.kg-1 Fe; 459 mg.kg-1 Mn; 2.163 mg.kg-1 Zn. Experiment 1 differed from the other experiments with regards to treatment layout. Experiment 1 included a foliar application of a 1%
phosphorous acid as a standard treatment with one application
two weeks before inoculation and another, one week after inoculation with P. cinnamomi. The uninoculated and inoculated
silicon treated trees were only treated twice, two weeks before
and one week after inoculation.
Experiment 2
Eighteen-month-old ‘Velvic’ avocado rootstocks from Schagen
nursery (Schagen, South Africa) grown in composted pine bark
were replanted in 5 L plastic pots in the same soil as experiment
1 and allowed to re-establish for eight weeks before the experiment was initiated.
Experiment 3
Twelve-month-old seedling ‘Duke 7’ avocado seedling rootstocks grown in composted pine-bark medium were acquired
from Westfalia Technological Services (Tzaneen, South Africa).
These trees were replanted in 5 L pots in steam-pasteurized soil
acquired from a soil supplier and allowed to re-establish for four
weeks before the experiment was initiated. The soil texture was
91% coarse sand, 4.4% silt, and 4.6% clay. The pH of the soil
used was 5.2 with 1800 ohm resistance and the chemical composition was 6 mg.kg-1 P, Bray I; 198 mg.kg-1 Ca; 41 mg.kg-1 K;
54 mg.kg-1 Mg; 23 mg.kg-1 Na; 2 mg.kg-1 Cu; 57 mg.kg-1 Fe; 31
mg.kg-1 Mn; 1 mg.kg-1 Zn.
Experiment 4
Eighteen-month-old ‘Velvic’ avocado rootstocks from Schagen
nursery (Schagen, South Africa) grown in composted pine bark
were replanted in 5 L plastic pots in the same soil as experiment
3 and allowed to re-establish before the experiment was initiated.
Treatments
Treatments consisted of
a) P. cinnamomi inoculated trees drenched with 20 ml.l-1 soluble
potassium silicate (20.7% silicon dioxide) at the rate of one
litre per tree as a once off application; or
b) multiple applications of potassium silicate (20.7% silicon dioxide) before and after inoculation (Bekker et al., 2006);
c) trees treated with potassium silicate and not inoculated;
d) inoculated trees treated with potassium phosphonate (Avoguard®);
e) trees inoculated and untreated;
f) and trees uninoculated and untreated (Table 1).
Ten replicate trees were assigned to each treatment and pots
were sorted according to a randomized block design on greenhouse benches to ensure even growth. Trees were grown in controlled environment greenhouses and watered manually every
second day with 300 ml water per pot.
Table 1: Treatments applied to avocado nursery trees grown in a greenhouse of three experiments to determine the effect of potassium silicate applications on Phytophthora cinnamomi root rot. Experiment 1 differed from the other experiments by having
a foliar application of a 1% phosphorous acid as a standard treatment, with one application two weeks before inoculation and
another one week after inoculation with P. cinnamomi. The uninoculated and inoculated silicon treated trees were only treated
twice, two weeks before and one week after inoculation with potassium silicate.
Treatment
Week 1
Week 2
Week 4
Week 7 Week 10 Week 13
Week 23
Silicon 1 day before inoculation
-
Silicon treated 1
day before Inoc.
-
-
-
-
Harvesting & Evaluation
Uninoculated, untreated control
-
-
-
-
-
-
Harvesting & Evaluation
Inoculated, untreated control
-
B
-
-
-
-
Harvesting & Evaluation
Inoculated & phosphorous acid
C
B
C
C
C
C
Harvesting & Evaluation
Silicon
A
-
A
A
A
A
Harvesting & Evaluation
Inoculated & silicon
A
B
A
A
A
A
Harvesting & Evaluation
A Application of 1 l of 20 ml.l-1 soluble silicon/pot
B Inoculation with P. cinnamomi
C Soil drench with potassium phosphonate (In experiment 1 this was a foliar application of a 1% phosphorous acid)
50
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
Inoculation procedure
An isolate of P. cinnamomi (freshly isolated from infected field
grown trees) was obtained from Westfalia Technological Services (Tzaneen, South Africa) and grown on potato dextrose agar
(PDA). Inoculum was prepared by soaking 300 g red millet seed
in 75 ml water for 24 h in 1 L Erlenmeyer flasks, whereafter 75
ml filtered V8 juice (Chen & Zentmyer, 1970; Cahill, Bennett, &
McComb, 1993) was added to the flasks. Flasks were then autoclaved twice for 45 min on two consecutive days, inoculated
with twenty P. cinnamomi culture (5 mm diameter) discs and
incubated for three weeks at 25°C. Four equidistant cylindrical
holes, 10 mm in diameter and 80 mm deep, were made in the
soil in each pot, at a distance of 50 mm from the stem of each
tree. Subsequently, 20 ml of P. cinnamomi millet seed inoculum
was placed in each hole, which was then sealed with soil and
watered thoroughly. This resulted in each tree receiving a total
of 80 ml inoculum.
Canopy condition
The canopy condition of each tree was rated according to a compiled rating scale from 1 to 5, with 5 = healthy looking tree and
1 = completely wilted / dead tree. Ratings were done independently by two parties, as well as from photographs taken during
harvesting (Figure 1). Leaves were counted per plant and leaf
area determined with a leaf area scanner (Licor1300, USA). Due
to the nutrient solution being too concentrated, leaves from these
experiments 2 and 3 showed signs of leaf tip burn and in severe
instances, leaf drop. Canopy ratings were therefore not done on
these trees in these two experiments.
Harvesting and evaluation
Trees were harvested after five (experiment 1) or 23 weeks (experiment 2, 3 & 4) and intact roots and shoots were photographed
for each plant. Root condition was assessed using a root rot rating scale of 1 to 5 (1 = roots completely rotten, with no root ball
present; 5 = no root rot, with a healthy intact root ball) and a root
regeneration rating scale of 1 to 5 (1 = no root regrowth; 5 = co-
a
b
pious new root-growth) (Figure 2). Representative photographs
were also taken of each treatment (Figure 3).
Re-isolation of P.cinnamomi from the trees after trial completion was only done for experiment 4. Ten root tips from each
plant were excised, rinsed in sterile, distilled water and plated
out on PARPH medium selective for Phytophthora. After incubation for seven days, the plates were examined microscopically
and P. cinnamomi identified. Data of experiment 4 is presented
in Table 2. Fresh mass was determined gravimetrically for both
roots and shoots of each plant. All plant material was dried in a
forced draught oven at 65ºC. Final dry mass was recorded for
roots and shoots of each plant and root : shoot mass ratios on a
dry mass basis were subsequently determined.
Table 2: Incidence of Phytophthora cinnamomi in the roots of avocado nursery trees either uninoculated or inoculated and treated
with soluble potassium silicate or potassium phosphonate of experiment 4. Values followed by the same letter do not differ significantly at 5% confidence interval.
Treatment
Incidence*
Silicon 1 day before inoculation
4.0b
Uninoculated, untreated control
0.0a
Inoculated, untreated control
9.0c
Inoculated & phosphorous acid
5.9b
Silicon
0.0a
Inoculated & silicon
5.2b
* From the ten root pieces plated out, the number of root pieces rendering positive
P. cinnamomi isolates
Data analysis
All data were analysed using Genstat® 4.23 DE for Windows®.
A general analysis of variance was performed for each data set
and means. Standard errors of the means and LSD’s at the 5%
confidence level were calculated.
c
d
e
f
Figure 1: Representative trees from the various treatments illustrating the canopy condition of avocado trees inoculated with P. cinnamomi
during experiment 4. From left to right: uninoculated, silicon treated tree (a); inoculated, potassium phosphonate treated tree (b); uninoculated, untreated tree (c); inoculated, untreated tree (d); tree treated one day before inoculation with silicon (e); and inoculated and silicon
treated tree (f).
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
51
5
4
3
2
1
Figure 2. Root rot assessment of harvested avocado trees according to a root rot rating scale of 1 to 5 (1 = roots completely rotten, with no
root ball present; 5 = no root rot, with a healthy intact root ball).
a
b
c
d
e
f
cinnamomi
Figure 3: Representative
ve samples of the root sy
system of avocado trees
tree
es inoculated with P. cinn
namomi and subjected to
na
ov
various treatments
from experiment 4. From left to right: uninoculated, untreated tree (a); inoculated, untreated tree (b); uninoculated, silicon treated tree (c);
inoculated, silicon treated tree (d); inoculated, potassium phosphonate treated tree (e); and a tree treated one day before inoculation with
silicon (f).
RESULTS AND DISCUSSION
Isolation frequency of P. cinnamomi from uninoculated, untreated control trees and trees treated only with silicon were zero,
indicating that the growth medium was free of pathogenic inoculum (Table 2). Root tips from inoculated, untreated trees had a
90% isolation frequency of P. cinnamomi, indicating the virulence
of the fungus as an inoculum. Root tips of inoculated and potassium phosphonate treated trees, inoculated, silicon treated trees
and trees treated with silicon applied one day before inoculation had statistically similar infection rates, indicating the effect of
silicon on root infection to be statistically similar to that obtained
through potassium phosphonate treatment. Trees receiving silicon one day before inoculation tended to have a lower incidence
of P. cinnamomi than that of inoculated and potassium phosphonate treated trees and inoculated, silicon treated trees, although
this was not statistically significant.
Root rot and regeneration
In experiment 1, trees that were inoculated with P. cinnamomi
and not treated with silicon or potassium phosphonate, had significantly more root rot than all other treatments (Table 3). P.
cinnamomi inoculated trees treated with either 1% phosphorous
acid (root rot rating = 4.67) or drenched with potassium silicate
and inoculated with P. cinnamomi (root rot rating = 4.60) were
statistically comparable to the uninoculated control (root rot rating = 5.00). Results indicated no significant differences in root
regeneration between treatments in experiment 1.
In experiment 2, root rot in trees that received a silicon application one day before inoculation (root rot rating = 1.20) did
not differ significantly from the inoculated untreated control (root
rot rating = 1.00). However, root rot in these two treatments
had significantly low ratings when compared to all other treatments. Roots from trees receiving only silicon (root rot rating =
Table 3: Effect of treatments with silicon and potassium phosphonate on root rot and root regeneration of Phytophthora cinnamomi inoculated avocado nursery trees in the greenhouse. Values in each column followed by the same letter do not differ significantly at 5%
confidence interval.
Clay
Experiment 1
Treatment
Sandy
Experiment 2
Experiment 3
Experiment 4
Root **
Root
Root
Root
Root rot*
Root rot
Root rot
Root rot
regeneration
regeneration
regeneration
regeneration
Silicon 1 day before inoculation
3.83b
1.33a
1.20a
2.20a
2.30b
2.00a
1.80a
2.50ab
Uninoculated, untreated control
5.00c
2.67a
3.9cd
1.40a
3.50c
2.38a
3.44b
4.00b
Inoculated, untreated control
3.33a
1.67a
1.00a
0.89a
2.10b
2.00a
1.66a
1.78a
Inoculated & potassium phosphonate
4.67c
1.33a
2.40b
2.20a
1.50a
1.10a
2.40a
3.30b
3.60ab
2.00a
4.20d
2.60a
3.20c
3.10a
3.80b
4.00b
4.60c
1.17a
3.30c
1.60a
2.88b
2.38a
1.70a
1.70a
Silicon
Inoculated & silicon
* Root rot assessed according to a rating scale of 1 to 5 (1 = roots completely rotten; and 5 = no root rot)
**Root regeneration assessed according to a rating scale of 1 to 5 (1 = no root regrowth; 5 = healthy new root formation)
52
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
4.20), and uninoculated, untreated tree roots (root rot rating =
3.90) had the lowest root rot rate. Root rot in inoculated, silicon
treated trees (root rot rating = 3.30) were low, and comparable
to root rot in uninoculated, untreated control roots. This rating
was significantly better than for phosphorous acid-treated trees.
Although there was no significant difference between treatments
with regards to root regeneration, there was a trend in silicon
treated trees as well as potassium phosphonate treated trees
to have healthier / uninfected roots compared to the inoculated,
untreated tree roots.
Root rot in potassium phosphonate treated trees was more
severe in experiment 3 (root rot rating = 1.50). Inoculated, silicon treated trees (root rot rating = 2.88), trees treated one day
before inoculation (root rot rating = 2.30), and inoculated, untreated trees (root rot rating = 2.10) were statistically comparable
with regards to root rot. Root rot of uninoculated, untreated trees
(root rot rating = 3.50) corresponded to that of silicon treated
trees (root rot rating = 3.20), and were the least affected by Phytophthora root rot. Although no significant differences were observed between treatments with regards to root regeneration,
silicon treated trees tended to have healthier roots compared to
other treatments.
There was not as marked a difference between treatments
in experiment 4, with regards to root rot, compared to other experiments. Both one (root rot rating = 1.80) and repeated silicon applications (root rot rating = 1.70) in conjunction with P.
cinnamomi inoculation did not result in an inhibition of root rot
development. Root rot in these treatments was statistically comparable to that of the inoculated, untreated control (root rot rating = 1.66). Root rot in uninoculated, untreated trees (root rot
rating = 3.44) and silicon treated trees (root rot rating = 3.80)
were statistically comparable, and less pronounced compared to
that of the inoculated, untreated trees. Regenerated roots were
more pronounced in uninoculated, untreated (root regeneration
= 4.00), inoculated and potassium phosphonate treated (root regeneration = 3.30) and silicon treated tree roots (root regeneration = 4.00) than the inoculated, untreated (root regeneration =
1.78), and inoculated, silicon treated trees (root regeneration =
1.70). Trees treated with silicon one day before inoculation with
P. cinnamomi did not differ significantly from any treatment with
regards to root regeneration.
In all experiments, root rot of the inoculated, untreated trees
were significantly more severe than that of the uninoculated, untreated trees, indicating the successful infection of nursery trees
after inoculation. Except for experiment one, root rot in trees
treated with silicon were statistically similar to root rot in uninoculated, untreated control trees and these trees had similar or
higher levels of root regeneration.
Soluble silicon polymerizes rapidly, resulting in insoluble
silicon compounds (Epstein, 2001). For effective disease suppression, silicon must therefore be applied continuously (Bowen
et al., 1995). This seems to be confirmed by results from the
present study, as trees receiving one silicon application one day
before inoculation did not exhibit improved resistance to Phytophthora root rot. Ghanmi et al. (2004) reported that although
the application of silicon to Arabidopsis thaliana prior to Erysiphe
cichoracearum D.C. inoculation did not prohibit fungal penetration and infection, the rate of disease development was altered.
In the current study, during the 23 weeks after inoculation,
no significant differences were obtained between the inoculated,
untreated control trees and those treated one day before inoculation with regards to root rot for experiments 2, 3 or 4. In experiment 1, where harvesting took place 40 days after inoculation,
root rot was more severe in the inoculated, untreated trees. It
could be that the disease developed slower in the once-off silicon treated trees, but this difference could not be detected 23
weeks after inoculation.
In the experiments conducted in the heavier soils (higher clay
content) (experiments 1 & 2), inoculated, silicon treated trees
showed statistically similar root rot ratings than the uninoculated
controls. However, in sandy soils (experiments 3 & 4) the trend
was different, and inoculated, silicon treated trees had significantly higher levels of root rot (a lower root rating) compared to
the uninoculated, untreated trees. This could be due to the cation
exchange capacity related to the clay percentage in each soil
type. The clay soil contained 21.3% clay, compared to the sandy
soil containing only 0.6% clay. Matichenkov and Bocharnikova
(2001) reported that soluble silicon compounds form complexes
with Al, Fe and organic compounds. However, if silicates from
siliceous-based fertilizers are not bound by the soil, these soluble nutrients leach from the plant available horizons, deeming
these elements unavailable for uptake (Tokunaga, 1991). In the
present study, it is believed that the applied potassium silicate
leached from the pots (in the sandy soils in experiment 3 & 4)
limiting the available silicon for plant protection and uptake. The
effect of applied silicates will therefore be more pronounced in
soils with high clay content.
Phosphonate fungicides, including potassium phosphonate,
fosetyl-Al and its breakdown product, phosphorous acid, are believed to control P. cinnamomi by a combination of direct fungitoxic activity and stimulation of host defence mechanisms (Guest
et al., 1995; Hardy et al., 2001) and is currently the preferred option of control of Phytophthora root rot in avocados (Hardy et al.,
2001). Silicon application inhibited Phytophthora root rot to levels similar to, or better than those obtained by potassium phosphonate applications. Wutscher (1989) reported that in young
orange trees, silicon accumulates in young leaves and feeder
roots, leading to protection of plant roots from infection. Root rot
data in the present study, however, tends to reiterate the findings
of Chérif et al. (1994) who stated that silicon deposited on the
surface of roots makes plant cells less susceptible to enzymatic
degradation by fungal pathogens. Application of silicon to partially resistant and susceptible rice cultivars to control leaf and
neck blast led to a decrease in disease severity levels similar to
those levels found in resistant cultivars not treated with silicon, or
better than that of commercial fungicide treated plants (Seebold
et al., 2000, 2004).
These findings are of paramount importance to the avocado
industry as it implies that potassium silicate may be proposed as
a possible alternative control for P. cinnamomi root rot on avocado nursery trees.
Canopy condition
‘Velvic’ rootstock trees grown in sandy soils and inoculated with
Phytophthora cinnamomi had lower canopy ratings (i.e. poorer
canopy conditions) than the uninoculated, untreated control trees
(canopy rating = 4.89) and uninoculated, silicon treated (canopy
rating = 4.7) trees (Table 4). Silicon and phosphonate treatments
of inoculated trees could not improve tree canopy health relative to the uninoculated, untreated control. Root rot of all the inoculated treatments were more severe than the uninoculated,
untreated control and uninoculated, silicon treated trees (Table
3). No significant differences could be seen between treatments
with regards to leaf area or number of leaves per plant. There
was, however, a trend present in terms of the number of leaves
per tree as uninoculated treatments generally had higher num-
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
53
ber of leaves than the inoculated treatments. This corresponds
with the report by Ploetz and Parrado (1988) who stated that a
moderate tolerance to Phytophthora root rot is often observed in
avocado trees where infection has occurred without degradation
of aboveground tree health. Reduced photosynthesis, transpiration and stomatal conductance can however be detected in root
rot affected trees before visible aboveground symptoms appear
(Sterne et al., 1978; Ploetz and Schaffer, 1989). Foliage becomes wilted and chlorotic, leaves fall and branches rapidly die
back depending on root rot severity (Ploetz and Parrado, 1988).
Results from the current study confirm this as canopy health assessment correlated with root rot severity.
Plant mass and root: Shoot ratios
Numerous physiological processes are affected by phytopathogens. Infected plants usually grow slower than corresponding
healthy plants and internodes are generally shorter. Once infect-
Table 4: Effect of silicon and potassium phosphonate treatments
on growth parameters of Phytophthora cinnamomi inoculated avocado nursery plants (cv. Velvic) in the greenhouse. Values in each
column followed by the same letter do not differ significantly at 5%
confidence interval.
Canopy
condition *
Av. Leaf
area (cm2)
No. of leaves
per plant
Silicon 1 day before
inoculation
3.70a
3749.40a
30.80a
Uninoculated, untreated
control
4.89b
3605.78a
41.11a
Inoculated, untreated
control
3.78a
3526.00a
35.56a
Inoculated &
K-phosphonate
3.70a
3178.60a
28.70a
Uninoculated, silicon
treated
4.70b
3889.80a
42.40a
Inoculated & silicon
3.60a
3137.40a
32.30a
Treatment
* Canopy condition assessed according to a rating scale (1 = permanently wilted
leaves; 5 = healthy leaves, no signs of wilting)
ed, most plants are less vigorous, have smaller root- and canopy
systems than healthy plants and leaf development is usually delayed (Russell, 1981). Because pathogens affect physiological
processes, including photosynthesis, it is likely that changes in
the amount of biomass and nutrients accumulated might also
occur. Ishiguro (2003) reported up to 67% root loss and 55%
aerial biomass loss due to Phytophthora cinnamomi infection of
oak and chestnut species. Plant growth, and especially carbon
partitioning between organs, is poorly understood and appreciable errors are made when estimating carbon partitioning as
a result of photosynthesis alone. Numerous other factors play
a role, including plant health and nutrient content of plant material ranging between 5-20% of the dry mass (Farrar, 1993).
Morikawa and Saigusa (2003) ascertained that if silicon was
added as a soil drench to blueberry (Vaccinum corymbosus cv.
bluecrop) cuttings, the silicon concentration in leaves of treated
plants were 85 times higher than any essential element, with a
mean concentration of 60 mg.g-1 dry weight. In the current study,
experiment 2 was the only experimental repeat that resulted in
significant differences between treatments with regards to root
mass (Table 5). Root fresh mass, experiment 2, of the inoculated, untreated control (24.08 g) was significantly lower compared to all other treatments. Fresh root masses of uninoculated,
untreated trees (39.58 g); inoculated, potassium phosphonate
treated trees (40.98 g); trees treated with silicon one day before
inoculation (43.42 g); and silicon treated trees (39.59 g) were
statistically similar to each other, but differed significantly from
the inoculated, silicon treated (58.11 g) trees with regards to
fresh root mass, the latter having the highest average fresh root
mass. Although not always statistically significant, results indicated the fresh root mass of inoculated, untreated trees to be the
lowest, compared to other treatments in all experiments except
for experiment 3, where the inoculated, potassium phosphonate
treated trees (59.46 g) had the lowest root fresh mass. This was
also true for root dry masses for all experiments except for experiment 3 where potassium phosphonate treated trees (15.50
g) had the lowest dry root mass compared to the other treatments. Inoculated, silicon treated trees showed the highest average fresh and dry root mass compared to all other treatments
for all four experiments, although this difference was not always
significant. This implies that silicon either stimulates growth or
Table 5: Effect of treatments with silicon and potassium phosphonate on root and shoot fresh (FM) and dry (DM) mass (g) of Phytophthora
cinnamomi inoculated avocado nursery trees in the greenhouse. Values in each column followed by the same letter do not differ significantly at 5% confidence interval.
Clay soil
Experiment 1
Treatment
Root
Shoot
DM
DM
Sand soil
Experiment 2
Root
FM
DM
Experiment 3
Shoot
FM
DM
Root
FM
Experiment 4
Shoot
DM
FM
DM
Root
FM
Shoot
DM
FM
DM
Silicon 1 day before
inoculated
11.50a 15.28c 43.42b 14.00a 47.16a 18.15a 77.34a 23.33a 42.47ab 17.99a 118.84a 49.67a 173.54ab 66.61a
Uninoculated,
untreated control
11.61a
8.96a
39.58b 17.89a 45.12a 18.32a 99.09a 27.75a 55.62b 20.52a 150.34a 64.29a 214.40b
Inoculated, untreated
9.78a
control
8.53a
24.08a 10.71a 46.69a 18.70a 65.24a 19.18a 50.16b 22.59a 113.60a 48.25a 172.23a 72.19ab
Inoculated &
K-phosphonate
11.85a 12.61b 40.98b 14.41a 54.61a 22.24a 59.46a 17.50a 29.27a 15.33a 135.16a 56.40a 160.56a
88.39b
64.43a
Uninoculated, silicon
12.10a 11.26ab 39.59b 15.22a 43.34a 18.56a 82.71a 21.16a 44.09b 17.80a 128.07a 50.59a 163.35a 68.27ab
treated
Inoculated & silicon
54
12.83a 13.46bc 58.11c 17.95a 55.30a 22.59a 107.56a 28.06a 43.59b 17.69a 172.29a 69.75a 204.25b
78.63b
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
imparts some form of protection to avocado roots if applied prior
to P. cinnamomi inoculation.
This protection has long been thought to be that of a physical barrier due to strengthening of the cell wall (Vance et al.,
1980; Aist, 1983; Nicholson and Hammerschmidt, 1992). However, recent evidence points towards the activation of an induced
systemic resistance (ISR) mechanism in the plant. Fawe et al.
(1998) proposed that silicon stimulates phytoalexin formation in
response to fungal attack. It could therefore be possible to further exploit this protection if soluble silicon is applied even earlier
than 10 days before inoculation.
There were no significant differences between the uninoculated, untreated (8.96 g) and inoculated, untreated controls (8.53 g)
with regards to leaf dry mass for experiment 1. These treatments
did not differ from the uninoculated, silicon treated (11.26 g) trees,
but were significantly different to all other treatments. Inoculated,
potassium phosphonate treated (12.61 g), uninoculated, silicon
treated (11.26 g) and inoculated, silicon treated (13.46 g) trees
did not differ with regards to leaf dry mass. In experiment 1, leaf
dry mass of trees treated with silicon one day before inoculation
(15.28 g) was however significantly higher than all other treatments. In experiment 3, leaf fresh mass of inoculated, potassium
phosphonate (29.27 g) treated trees was statistically comparable
to trees treated with silicon one day before inoculation (42.47 g),
but significantly lower than all other treatments.
Uninoculated, untreated control trees (214.40 g) and inoculated, silicon treated trees (204.25 g) in experiment 4 were significantly higher when compared to all the other treatments with
regards to leaf fresh mass. With regards to root dry mass, trees
(experiment 4) treated with silicon one day before inoculation
(66.61 g) were statistically similar to inoculated, potassium phosphonate treated trees (64.43 g), but significantly different from
uninoculated, untreated control (88.39 g) and inoculated, silicon
treated (78.63 g) trees. Leaf dry mass of inoculated, untreated
control (72.19 g) and silicon treated (68.27 g) trees did not differ
from any treatment.
Although differences between treatments were not consistently significant, leaf fresh mass of experiments grown in sandy
soil were the highest in uninoculated, untreated controls, whilst
the inoculated, potassium phosphonate treated trees resulted
in the lowest leaf fresh mass. For experiments grown in sandy
soils, inoculated, potassium phosphonate treated trees had the
lowest leaf dry mass compared to the other treatments.
In experiment 1, the root : shoot dry mass ratio of inoculated,
silicon treated trees (1.77) was significantly higher that all other
treatments (Table 6). There were no other significant differences
between treatments with regard to root : shoot mass ratios between all treatments and in both soils. Root : shoot ratios were
generally higher in sandy than in clay soils.
Sterne et al. (1977) reported the effect of soil structure on
Phytophthora root rot disease development to be determinant
of the level of disease severity. This in turn creates an imbalance in the source-sink relationship between plant parts. Higher
root : shoot ratios indicate a healthy root system. In the current
study, clay soils led to lower root compared to leaf masses,
but in contrast, trees grown in sandy soils did not experience
such a high level of root rot, leading to higher root : shoot ratios.
These results corroborate the statements made by Sterne et al.
(1977), suggesting a heightened disease combating strategy to
be implemented in avocado orchards situated in soils containing
high clay percentages, as clay soils have greater water retention
properties which aid in the hastily spread of the disease.
CONCLUSION
Potassium silicate application to Phytophthora cinnamomi infected trees resulted in effective inhibition of root rot, similar to
levels obtained by commercial application of potassium phosphonate (Avoguard®). Potassium silicate application imparts protection to roots under infection pressure, and induces new root
growth. The beneficial effect of potassium silicate is, however,
dependant on reapplication, as these beneficial effects are lost
if control is reliant on only one application. The timing of reapplication will be determined by, amongst other factors, the growth
medium characteristics, as silicon leaches easily in media with
low CEC, rendering the applied silicon as unavailable for plant
uptake. Sandy soil will therefore necessitate more regular applications of silicon to maintain the level of disease suppression
reached in the host plant.
Root rot of inoculated trees treated with silicon were, in all
experiments, either statistically comparable to, or better than
root rot in inoculated trees treated with potassium phosphonate
(the standard commercial fungicide), implying that silicon does
induce some form of resistance in the plant suppressing fungal
penetration and infection. These findings are of paramount importance as this implies that potassium silicate may be proposed
as an alternative control to inhibit the effects of P. cinnamomi on
avocado trees.
Silicon treated trees had the highest fresh and dry root mass
compared to all other treatments. This implies that silicon either
stimulates growth or imparts some form of protection to avocado
roots if applied prior to P. cinnamomi inoculation. Leaf fresh mass
Table 6: Effect of treatments with silicon and potassium phosphonate on fresh and dry root : shoot (R:S) mass rations of Phytophthora
cinnamomi inoculated avocado nursery trees in the greenhouse. Values followed by the same letter do not differ significantly at 5% confidence interval.
Clay soil
Experiment 1
Sand soil
Experiment 2
Experiment 3
Experiment 4
R:S fresh
R:S dry
R:S fresh
R:S dry
R:S fresh
R:S dry
R:S fresh
R:S dry
Silicon 1 day before inoculation
1.14a
0.96a
1.00a
0.80a
1.91a
1.30a
1.64a
0.72a
Uninoculated, untreated control
0.72a
0.81a
1.02a
0.99a
1.81a
1.32a
1.24a
0.86a
Inoculated, untreated control
1.41a
0.94a
0.58a
0.62a
1.41a
0.87a
1.31a
0.90a
Inoculated & K-phosphonate
1.18a
1.10a
0.76a
0.67a
2.61a
1.18a
1.35a
0.73a
Uninoculated, Silicon treated
1.37a
1.05a
1.31a
0.87a
2.41a
1.63a
1.52a
0.73a
Inoculated, Silicon treated
1.04a
1.77b
1.05a
0.87a
1.88a
1.17a
1.61a
0.66a
Treatment
SOUTH AFRICAN AVOCADO GROWERS’ ASSOCIATION YEARBOOK 30, 2007
55
of inoculated, silicon treated trees was similar to that of uninoculated, untreated trees. For experiments grown in sandy soils,
inoculated, potassium phosphonate treated trees resulted in the
lowest leaf dry mass compared to all the other treatments.
Drawing on this knowledge, where P. cinnamomi infection is
already prevalent in the field, it is expected that protection of
large trees, as a result of drenching the soil with soluble silicon,
would be incremental. In previous studies it has been proposed
that silicon increases diffusive resistance, or decreases the effect of infection on diffusive resistance, and if therefore applied
after infection, may lead to increased diffusive resistance over a
longer period of time.
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